Membrane distillation for desalination involves the passage of hot salt water (as a liquid feed) over a microporous (e.g., having an average pore size of approximately 0.2 μm) hydrophobic membrane that allows pure water vapor through while retaining the dissolved salts in solution by establishing a temperature-driven vapor-pressure difference between the feed and permeate sides of the module. The hydrophobicity of the membrane ensures that liquid water does not pass through the membrane and thereby ensures nearly complete elimination of non-volatile impurities.
Depending on the design of the condensing/permeate system on the other side of the membrane, membrane distillation is categorized into various types, as described below.
In direct-contact membrane distillation (DCMD), a cold pure water stream flows on the other side (i.e., the permeate/condensate side) of the membrane from and counter-current to the feed; and the water vapor condenses into the cold pure water stream (transferring heat into the cold pure water stream) when the water vapor leaves the membrane. Because the hot and cold streams are separated only by a thin membrane, there is significant sensible heat transfer. This heat transfer, in addition to being a loss, also adds to temperature polarization in the streams. The heated pure water stream then passes through a heat exchanger where energy is transferred into the incoming feed to preheat it, thereby recovering part of the condensation energy.
In the case of air-gap membrane distillation (AGMD), there is an air gap across which the vapor diffuses before condensing on a heat-transfer plate maintained at a low temperature by a coolant stream. Consequently, sensible heat loss from the feed is reduced since air has a lower thermal conductivity. The evaporated water has to diffuse through the air gap and reach the film of condensate on the heat-transfer plate, which becomes one of the rate-limiting steps. A liquid feed is circulated via a pump and flows through respective channels on opposite sides of the membrane and the heat-transfer plate and is heated by a heater when passing from the first channel (where the liquid feed serves as a coolant via heat transfer through the heat-transfer plate) to the second channel, from which the pure water is removed from the heated feed water through the membrane. Pure water product is extracted from the bottom of the air gap, while the brine remaining from the liquid feed is extracted from the bottom of the second channel. In other embodiments, the pure water product is extracted from the top of the gap forcing the air gap to be flooded with water, forming a liquid gap. The gap usually contains a spacer material, typically made of non-conductive plastic to hold the membrane in place.
Permeate-gap membrane distillation (PGMD), where the gap is filled with condensate, without the high conductivity spacers has been suggested as a superior alternative to both air-gap (AGMD) and direct-contact (DCMD) configurations. Usually, the gaps in AGMD and PGMD systems have also used a spacer material, often made of plastic to hold the membrane in place.
Sweeping-gas membrane distillation (SGMD) utilizes an air stream that flows on the permeate side picking up the incoming vapor and becoming humidified as the stream moves along the module. Generally, the temperature of the air also increases along the module. The hot humid air is then cooled in a condenser where produced pure water is recovered.
Material-gap membrane distillation (MGMD) is a recent configuration where materials, such as sand, which have a low thermal conductivity (i.e., act as a thermal insulator), are used to fill the gap [see L. Francis, et al., “Material gap membrane distillation: A new design for water vapor flux enhancement,” 448 Journal of Membrane Science 240-247 (2013)].
Other configurations include the vacuum membrane distillation (VMD) system, which has been adapted to produce a multi-stage configuration and is being marketed commercially by Memsys of Singapore and Germany [see Zhao, K., et al., “Experimental study of the Memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module,” Desalination 323 (2013): 150-160], and the conductive-gap membrane distillation (CGMD) system, wherein a thermally conductive material is mounted in the gap or the gap is made extremely thin with grooves or fins on the condensing surface to facilitate heat transfer across the gap.